The present invention relates to gas assisted compression molding of plastic parts. Various plastic molding systems such as injection molding and compression molding are known in the art. Generally, in injection molding, hot molten plastic resin is injected to flow into a closed and sealed mold. This process has been modified to inject gas into the mold to push the molten resin to completely and uniformly cover the surfaces of the mold. Compression molding begins with a solid charge of moldable material, which may be in the form of plastic or any polymeric compound which is introduced in solid form into the inner cavity of the lower portion of an open mold. The plastic charge is then heated and the mold is closed thereby compressing the plastic into the particular shape desired for the finished part. The compression caused by the mold closure forces the plastic throughout the mold and against the mold surface. No known attempt has been in the art to use gas to assist in this distribution of plastic in a compression molding application. Typically, to ensure complete fill out of the mold, enough plastic must be placed in the mold to completely fill the mold volume. In fact, compression molds are typically provided with overflow chambers at the extremities of the mold and excess plastic introduced to ensure that even the extremities are completely filled. However, even if the mold cavity is completely filled, the plastic material, which is either intentionally heated to facilitate its flow or increases in temperature due to the compression, will shrink upon cooling. This shrinkage or sagging causes the plastic to pull away from the top mold surface. Typically, the top mold surface is the xe2x80x9cBxe2x80x9d surface or xe2x80x9cbackxe2x80x9d of the compression molded part and appearance is not critical.
As molding techniques have improved, tolerance for a less finished appearance for the xe2x80x9cBxe2x80x9d side has decreased. Further, although compression molding is more cost effective in many applications, it has been avoided because of a perceived inability to mold with uniformly finished sides. Still further, compression molding has often been deemed impractical for the production of certain parts because of difficulty in compressing the resin into certain shapes as the plastic does not flow as far or fluidly into narrow mold cavities as the more molten resin of injection molding. This condition is complicated or compound by the use of fillers in the plastic charge.
Since compression molding utilizes a prepared plastic charge or billet placed directly in the cavity, there has not been a perceived need for the molten plastic to be able to flow through relatively small orifices such as an injection molding sprue; and thus the plastic used may contain much less uniform materials than in other plastic molding materials. For example, compression molding plastic can contain fillers such as scrap plastic to reduce cost or elongated glass fibers to improve strength. Wherefore, there is a need in the art to increase the scope and efficacy of the compression molding process to allow use of such materials in a wider range of applications. One attempt to improve the speed and uniformity of the melting of the compression molded plastic in a mold is to heat the moldable material by introducing hot air into the then unsealed mold which is vented near the extremities before compression (U.S. Pat. No. 5,714,455).